1. Statement of the Technical Field
This document relates to systems employing Digital Selective Calling (“DSC”). More particularly, this document concerns systems and methods for space-based DSC.
2. Description of the Related Art
Automatic Identification Systems (“AISs”) are well known in the art. The AISs typically allow vessels (e.g., ships) to view and track marine traffic in a surrounding area. AISs have many applications. For example, AISs can be employed for collision avoidance, fishing fleet monitoring and control, vessel traffic services, maritime security, navigation services, search and rescue, accident investigation, and fleet and cargo tracking.
In this regard, an AIS is an automatic tracking system used on ships and by Vessel Traffic Services (“VTSs”) for identifying and locating vessels in a given geographic area or around the globe. A vessel's identification and location are tracked by exchanging data with other nearby vessels, AIS base stations and satellites. The vessel's identification and location are displayed in an AIS chartplotter or other Graphical User Interface (“GUI”) viewable on a display screen. The AIS chartplotter and other GUIs facilitate collision avoidance amongst a plurality of vessels in proximity to each other. Other information may also be displayed on the display screen, such as a vessel's position, course and/or speed.
The vessels comprise AIS transceivers which automatically and periodically transmit vessel information. The vessel information includes, but is not limited to, vessel name, position, speed and navigational status. The vessel information can be used to track the vessel by the AIS base stations and/or satellites. The AIS transceivers comprise a Very High Frequency (“VHF”) transceiver and a positioning system (e.g., a Global Positioning System (“GPS”)). The VHF transceiver has a VHF RF range of about 10-20 miles in conjunction with a Self-Organized Time Division Multiple Access (“SOTDMA”) scheme.
The vessels and coastal stations also have a DSC capability. In this regard, each of the vessels and coastal stations consists of a VHF DSC transceiver. The VHF DSC transceiver facilitates distress related communications over terrestrial marine radio systems. For example, in the event of an emergency, the VHF transmitter is used to send an automatically formatted distress alert signal to surrounding vessels and/or coastal stations of rescue authorities. The distress alert signal can include a designation specifying the nature and priority level of the call, a vessel's unique identifier (e.g., Maritime Mobile Service Identity (“MMSI”)), and the vessel's position. In response to a reception of a distress alert signal, vessels and/or coastal station(s) send a DSC acknowledgement message to the vessel that transmitted the distress alert signal. The DSC acknowledgement message is received by the VHF DSC receiver. Thereafter, the AIS transceiver tunes to a designated channel for further distress related communications.
Despite the advantages of the DSC capability, it suffers from certain drawbacks. For example, there is minimal to no DSC coverage in the polar regions of the globe. As such, oil exploration ships exploring the polar regions of the globe cannot rely on a DSC capability for distress related communications. Additionally, there is no DSC capability when vessels operate beyond the RF range of DSC-equipped coastal regions and nearby vessels (e.g., open oceans).